1. Field of the Invention
The present invention relates to a composition for film formation, which can be formed into a porous film that excels in dielectric properties, adhesion, film consistency and mechanical strength, and has reduced absorption; a porous film and a method for forming the same; and a semiconductor device, which contains the porous film inside.
2. Description of the Related Art
The porous silica material which is produced through hydrolysis and polycondensation of the silica precursor in the presence of micelle has attracted attention because it has a narrow pore distribution with nanometer size, high regularity of structure and extremely high mechanical strength. The application of support for various catalysts and others has been being studied.
According to Japanese Patent Provisional Publication No. 2001-157815, this porous silica material is applied for a moisture absorbent because it is generally extremely hygroscopic and can absorb water vapor even under low relative vapor pressure. However, the application of the porous silica material as the material having the low dielectric constant has a problem because its hygroscopic property leads to high dielectric constant. Therefore, the haste measure has to be taken to correct the problem.
According to one of reported methods of preventing moisture absorption, as soon as the silica porous body medium is formed, its surface is subjected to moisture-preventing coating. In another method, the formed silica porous body is reacted with a silylation agent such as hexamethyldisilazanes so that a trimethylsilyl group is introduced on the surface of the silica porous body for enhancing hydrophobicity.
Although these methods can prevent the silica film from absorbing moisture, it is necessary that these treatments have to be performed as soon as the silica film is formed. Accordingly, these methods result in complicated process. Moreover, complicated operations such as adjusting the atmosphere during the treatment and storage are required.
In the method using hexamethyldisilazan, influence of the possible amine residue on the subsequent process is concerned. Thus, a method for obtaining a hydrophobic porous silica film which does not require said hydrophobic treatment has been sought.
Moreover, Sandra L. Burkett et al. proposed the method for producing hydrophobic silica bulk by hydrolyzing the composition contain phenyltriethoxysilane or n-octyltriethoxysilane in tetraethoxysilane in the presence of ammonium salt micelle. However, the n-octyl product which is expected to have more hydrophobicity cannot maintain its shape during the removal of the ammonium salt by washing with ethanol containing hydrochloric acid (J. Chem. Soc., Chem. Commun., 1996, 1367). The phenyl product is contained in a large portion such as 10 wt % and 20 wt % so that it seems to have made not only the surface but also the silica itself hydrophobic.
As mentioned above, it is very difficult to prevent the silica porous body from absorbing moisture. When the porous film made of the silica porous body is used as an insulator film in multi-level interconnects of the semiconductor device, there are problems such that moisture-absorbed porous film increases dielectric constant and lowers the adhesiveness in the lamination of films. Moreover, when the dielectric constant of the porous film used as an insulator film increases, the RC delay in the multi-level interconnects of the semiconductor device is increased. Consequently, the performance of the semiconductor device (high speed and low power consumption) has not been improved. Furthermore, when conventional porous film is incorporated into multi-level interconnects, there is a problem of the film having insufficient mechanical strength. The low mechanical strength results from the porous film property. Thus, there is a problem that low mechanical strength of the porous film as an insulator film lowers the reliability of a semiconductor device.